The present invention relates to a liquid level detecting device. In detail, the invention relates to a liquid level detecting device that automatically detects a residual amount of a liquid stored in a fuel tank for transportation of an automobile, an air plane or the like on the basis of a position of a liquid level of the liquid.
In the past, as a liquid level detecting device that detects a height of a liquid level in a fuel tank of, for example, an automobile, one is known which is adapted to detect a height of a liquid level in such a manner that a float arm is slid on a resistor plate by a float which vertically moves in association with the liquid level of the liquid and the liquid level is converted into an electric potential.
Here, an example of a liquid level detecting device is described below. FIG. 1 is an electric block diagram for explanatorily showing a structural example of a sensor used in a liquid level detecting device according to the invention and the related art. FIG. 2 is a schematic view explanatorily showing a structural example of the liquid level detecting device according to the invention and the related art. FIG. 3 is a schematic view explanatorily showing a structural example of a variable resistor in the sensor according to the invention and the related art.
A sensor 2 of a liquid level detecting device 1 has a variable resistor 3 of which the resistance value is changed in a process in which contact points 19 and 20 (described later) move in association with variation in height of a liquid level in a liquid-tight container T. The variable resistor 3 is serially connected to a fixed resistor 7 and is connected to a power supply circuit 4 that applies a predetermined voltage to the variable resistor 3 and the fixed resistor 7.
As shown in FIGS. 2 and 3, the sensor 2 has a resistor plate 13 attached to a body frame 12 and a sliding unit contact element 14 attached to the other end of a float arm 11 having, attached to its tip portion, a float 10 capable of floating at a liquid level by a buoyant force of a liquid. The resistor plate 13 of the sensor 2 is provided with a first conductive pattern 15 and a second conductive pattern 16. The two conductive patterns 15 and 16 are arranged such that they are formed in arc shapes centering around a rotation shaft 21 of the float arm 11 so as to be parallel to each other. A conductive section 17 for inputting/outputting is connected to one end of the first conductive pattern 15 and a conductive section 18 for inputting/outputting is connected to one end of the second conductive pattern 16.
The first conductive pattern 15 is formed of a plurality of conductive segments 15a which are arranged in the circumferential direction of the arc shape at a predetermined interval and a resistor 15b which mutually and electrically connects the plurality of conductive segments 15a. In addition, the second conductive pattern 16 is formed of a plurality of conductive segments 16a which are arranged in the circumferential direction of the arc shape at a predetermined interval and a resistor 16b which mutually and electrically connects the plurality of conductive segments 16a. 
The sliding unit contact element 14 is provided with two contact points 19 and 20 which are electrically connected to each other. In addition, a rotation shaft 21 positioned at the other end of the float arm 11 is coupled to the sliding unit contact element 14. The float arm 11 is rotated in the direction of arrow Y in FIG. 3 about the rotation shaft 21 as a fulcrum in association with movement of the float 10 floating at a liquid level, the float 10 being moved downward from a position of the liquid level in a full-up state corresponding to an amount of a consumed liquid. In association with the rotation of the float arm 11, the sliding unit contact element 14 is also rotated in the direction of arrow Y in FIG. 3. By the rotational movement of the sliding unit contact element 14, the contact points 19 and 20 are electrically brought into contact with the respective conductive segments 15a and 16a arranged on the first conductive pattern 15 and the second conductive pattern 16 while sliding on the respective conductive segments 15a and 16a. With this, the length of the resistor 15b interposed in a circuit provided between the conductive section 17 for inputting/outputting connected to the first conductive pattern 15 and the conductive section 18 for inputting/outputting connected to the second conductive pattern 16, is changed so that a resistance value of the circuit is change (That is, the resistance value of the variable resistor 3 in FIG. 1 is change). Thus, the variable resistor 3 is formed of the first conductive pattern 15, the second conductive pattern 16 and the sliding unit contact element 14.
A voltage is applied to the variable resistor 3, the sensor 2 detects an electric potential difference between the conductive section 17 for inputting/outputting and the conductive section 18 for inputting/outputting and outputs the detected signal to a processing circuit 5, and the processing circuit displays a residual amount of the liquid on a display device such as a meter 6 or the like by an analogue or bar graph on the basis of the output signal of the sensor 2. Meanwhile, a fixed resistor can be disposed on a wiring line with the processing circuit 5 in the meter 6.
In such a liquid level detecting device, a silver-palladium (AgPd) alloy, a silver-copper (AgCu) alloy, a silver-Nickel (AgNi) alloy, or the like is generally used for a material of the contact point. The conductive segment is formed of, for example, a mixture of powders of silver-palladium (AgPd) and glass, which is obtained in such a manner that silver powders, palladium powders and glass powders are mixed to produce a mixture in pasty form, the mixture in pasty form is printed on a resistor plate to be dried, and then it is baked.
Meanwhile, a liquid level detecting device is sometimes used for a fuel tank of an automobile which uses, as a fuel, an electrolysis solution (alcohol) itself such as ethanol, methanol or the like, or gasoline containing the electrolysis solution. While silver (Ag) has a small electric resistance and superior conductivity, the contact point or the conductive segment is deteriorated or corroded by a sulfur content, moisture, alcohol or the like in the fuel so that a malfunction such as impossibility of measuring, detection of a wrong value, or the like possibly occurs due to a failure of conduction. Further, in the recent fuel circumstances over the world, there are many chances that a fuel having various compositions is used so that it is necessary to prevent the above malfunction so as to provide a fuel meter having a reliability. With that, a technique is known that a part on which a contact point of a conductive segment is slid, is covered with an alloy containing gold (Au) in order to prevent the conductive segment or the contact point from being deteriorated or corroded (See, for example, JP-A-2003-287456 and JP-A-2009-162694).
In accordance with techniques of JP-A-2003-287456 and JP-A-2009-162694, while there is effectiveness in antideterioration and anticorrosion of the conductive segment, a cover layer becomes thin with the lapse of time so that continual effectiveness is possibly insufficient. In addition, in order to ensure the sufficient antideterioration and anticorrosion, it is necessary to cause the conductive segment to contain much gold (Au) (for example, approximately 40% by mass or more in the conductive segment) so that a problem arises that the cost is increased.